1. Field of the Invention
This invention relates to a process for protecting the environment, the process involving treatment of an industrial, cyanate-containing waste stream so that it can be discharged into receiving waters without potentially harmful effects. More particularly, the invention concerns such a process in which the waste is treated to convert cyanates to urea and to decrease the ammonia content of the treated waste to a concentration that will not render the receiving waters harmful to aquatic species.
2. Description of the Prior Art
Cyanides, in the form of cyanide ions, hydrogen cyanide, metallocyanides and metallocyanide complexes, are used in a variety of industrial processes. For example, aqueous solutions of such cyanides are used to leach gold and silver directly from ores. Cyanides are also used in processes for electroplating gold, silver, platinum, iridium, copper, zinc, and the like. Such processes generate significant quantities of highly toxic cyanide waste. It is conventional to oxidize the waste cyanides to cyanate salts or cyanic acid (collectively referred to herein as "cyanates"), which are much less toxic than the cyanides from which they were derived. Because the cyanates are of low toxicity to mammalian and aquatic species, cyanate-bearing waste materials are often discharged at low concentrations (e.g., 0.2 gram/liter) into receiving waters (e.g., rivers, lakes and the like) where they are further diluted by a factor of ten or more.
Heretofore, the ultimate fate of cyanate-bearing materials discharged into receiving waters has generally been of little concern to environmentalists. However, the present inventor recognized that cyanates can generate ammonia and ammonium ions, that cyanate-bearing wastes often contain additional quantities of ammonia from other sources and that the presence of ammonia can be a serious environmental concern because ammonia can be toxic to fish even at very low concentrations.
Ammonia, ammonium ions and other ammoniacal residues (referred to collectively herein as ammonia) usually are present in cyanate-bearing waste streams, at concentrations of about a tenth to a hundredth of the cyanate concentration. Several possible sources contribute to the ammonia content of cyanate-bearing waste. These sources include cyanate decomposition, cyanide hydrolysis during processing of the ores and ammonium nitrate from explosives that were used during mining of the ores.
A conventional method for handling aqueous cyanate-bearing wastes often involves impounding the waste for long times in ponds. In the ponds, the cyanate is very slowly converted to urea and ammonia gas slowly escapes from the surface of the pond. In acidic ponds, the escape of ammonia is particularly slow.
An object of this invention to provide a process that rapidly converts aqueous wastes which contain cyanates and lesser amounts of ammonia into materials that can be safely discharged to receiving waters without potential harm to aquatic species.